Massive stars observed in the Small Magellanic Cloud. Of the stars studied, seventy percent (the orange diamonds) appear to accelerate and decelerate, indicating the presence of a companion. Credit: ESO/Sana et al.
Saint Mary’s University Astronomer Dr. Vincent Hénault-Brunet is part of a team of researchers who observed that massive stars in metal-poor galaxies often live in pairs
In humans, twins or higher multiples are relatively rare, with single births being the norm. For massive stars, however, the opposite seems to be true: most are born and live as pairs. This was already known for massive stars in our own metal-rich Milky Way, but new observations reveal that massive stars in metal-poor galaxies also frequently have close companions.
Dr. Vincent Hénault-Brunet
The discovery was made by the BLOeM (Binarity at LOw Metallicity) collaboration, an international team of 70 astronomers that includes Dr. Vincent Hénault-Brunet from Saint Mary’s University. Using the European Very Large Telescope in Chile, the team monitored the velocities of massive stars in the Small Magellanic Cloud. Their findings are published in Nature Astronomy.
In recent years, astronomers have realized that interactions between stellar companions play a crucial role in the evolution of massive stars in the Milky Way. Until now, however, it was unclear whether massive stars in more distant, metal-poor galaxies were also part of binary systems.
“We used the Small Magellanic Cloud as a time machine,” explains Hugues Sana from KU Leuven (Belgium), the lead author of the study. “The Small Magellanic Cloud is a relatively nearby metal-poor galaxy, an environment representative of that of much more distant galaxies when the universe was only a few billion years old.”
Studying massive stars outside of the Milky Way is difficult because the stars are far away and we receive little light from them. The researchers used the FLAMES spectrograph on the Very Large Telescope of the European Southern Observatory in Chile. It is one of the largest telescopes on Earth. FLAMES has 132 fiber optics, each of which can be directed at a different star, which can then be observed simultaneously.
Over a period of 3 months, the researchers observed the acceleration and deceleration of 139 massive O-type stars at 9 different times. These stars have masses between 15 and 60 times that of our Sun. They are hot, shine brightly, and end their lives in supernova explosions. In the process, the star's core collapses into a black hole. The results show that over 70 percent of the observed stars accelerate and decelerate. That is a sign of a close companion.
“The fact that massive stars in the Small Magellanic Cloud have a partner suggests that the first stars in the universe, which we suspect were also massive, had partners, too," says co-author Julia Bodensteiner of the University of Amsterdam (the Netherlands). "Perhaps some of those systems end up as two black holes orbiting each other. It’s an exciting thought."
These black holes are of particular interest to Dr. Vincent Hénault-Brunet and his research group at Saint Mary’s, who study their subsequent evolution in dense clusters long after their progenitor massive stars have exploded as supernovae.
Scientific article: A high fraction of close massive binary stars at low metallicity. By: Hugues Sana, Tomer Shenar, Julia Bodensteiner, et al. In: Nature Astronomy, 2 September 2025. [original | preprint (pdf)]